enetc: Fix a signedness bug in enetc_of_get_phy()

The "priv->if_mode" is type phy_interface_t which is an enum.  In this
context GCC will treat the enum as an unsigned int so this error
handling is never triggered.

Fixes: d4fd0404

 ("enetc: Introduce basic PF and VF ENETC ethernet drivers")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>